(1) In a double boiler, put 200g dark cholocate and 125 g unsalted butter in the upper pot, heat under medium power, stir until the mixture is smooth and silky;

(2) Remove the upper pot from heat, let cool to room temperature;

(3) Line springform pan (9') with parchment paper;

(4) Beat 4 yolks with 200g (minus 1 tbsp) sugar until it turns white in color and becomes very thick;then whisk in chocolate mixture;and mix 100 g sifted cake flour, one tbsp at a time, until the batter is very smooth;